Page:Descent of Man 1875.djvu/318

302 transition takes place by their becoming confluent and straight, and at the same time more prominent and smooth. A hard ridge on an adjoining part of the body serves as the scraper for the rasp, but this scraper in some cases has been specially modified for the purpose. It is rapidly moved across the rasp, or conversely the rasp across the scraper.



These organs are situated in widely different positions. In the carrion-beetles (Necrophorus) two parallel rasps (r, fig. 25) stand on the dorsal surface of the fifth abdominal segment, each rasp consisting of 126 to 140 fine ribs. These ribs are scraped against the posterior margins of the elytra, a small portion of which projects beyond the general outline. In many Crioceridæ, and in Clythra 4-punctata (one of the Chrysomelidæ), and in some Tenebrionidæ, &c., the rasp is seated on the dorsal apex of the abdomen, on the pygidium or pro-pygidium, and is scraped in the same manner by the elytra. In Heterocerus, which belongs to another family, the rasps are placed on the sides of the first abdominal segment, and are scraped by ridges on the femora. In certain Curculionidæ and Carabidæ, the parts